1. Field of the Invention
The present invention relates to the manufacture of composite metal parts which have a contact surface intended to withstand abrasion.
2. Description of the Prior Art
So-called mechanical "wearing" parts are known which are superficially reinforced by adding a material which has improved properties in terms of resistance to wear by abrasion.
Means commonly used to obtain a reinforced contact surface include techniques of adding hard alloys by welding. These techniques are usually referred to by engineers as "hardfacing".
The present invention relates in particular to hardfacings which use a structure of extremely hard grains which are bonded to each other by a metal alloy, commonly referred to as a metallic matrix.
Hardfacing by welding may use a wide variety of processes and materials. It is convenient to use hardfacing with a tungsten carbide or other extremely hard material base. For instance, rigid or flexible welding rods are used; one end is applied to the surface to be hardfaced and is then subjected to an electric arc or an oxy-acetylene flame. The welding rod contains tungsten carbide powder mixed with a nickel-based alloy or other appropriate metals.
Such hardfacing using tungsten carbide based welding rods has a certain number of drawbacks.
In particular, welding processes mean that the hardfacing is deposited in the form of successive side-by-side beads. It is obvious that producing a relatively large surface area using such a technique is time-consuming and requires a certain amount of skill and dexterity on the part of the operator.
The waves inherent in such a deposition process in the form of successive beads cause irregularities in thickness which may be as much as several millimeters. As a result, this technique cannot be used to produce parts having accurate dimensions. In addition, this technique cannot be used to produce parts which have complex shapes, e.g. those that have grooves and ribs, hollows, sharp edges. Welding irregularities are greater if the parts to be hardfaced have complex shapes.
A process is known from document EP-A-0 123 961 which is similar to traditional casting processes: in the initial stage, a molten binder metal is poured into a cold sand mould; in a subsequent stage, a hard powder is sprinkled onto the molten metal in the mould, drops to the bottom of the mould and forms the hard layer which is continuously bonded to the upper layer of molten alloy. Such a process is difficult to use and does not make it possible to control the density and shape of the abrasion-proof layer thus produced.
Document GB-A-2 003 932 also discloses a moulding process by melting powder in a mould. The moulds described in this document are made of graphite or a molybdenum alloy.
In document FR-A-2 160 117, a process has been disclosed to coat internally a cylinder by centrifugal moulding. The cylinder is filled with a binder alloy and particles of a hard material. This assembly is heated to a high temperature in a furnace. At the start of cooling, the cylinder is rotated at high speed to force the hard particles to move outwards in the direction of the cylinder wall. After cooling, the inside of the cylinder is machined to obtain the desired diameter.
Processes for producing a layer of a composite abrasion-proof material which use infiltration techniques are also known, e.g. the technique described in French patents FR-A-1 398 732 and FR-A-2 352 890. These techniques use a cavity mould made of carbon or a ceramic material which has the desired shape; a core is placed in the cavity of the mould opposite the walls of the mould; the internal space between the core and the walls of the mould is filled with grains of tungsten carbide or equivalent and the assembly is vibrated to compact the grains; grains or pellets of binder metal or alloy are placed on top of the tungsten carbide grains; the assembly is heated to a temperature which exceeds the melting point of the alloy but is less than the melting point of the core and the mould: this increase in temperature melts the binder alloy or metal which spreads into the space filled with tungsten carbide grains and welds with the core if the latter is made of metal. The assembly is allowed to cool before stripping.
This infiltration technique is relatively suitable for parts which have flat abrasion-proof surfaces. The mould can then be produced at low cost without requiring complicated machining. In contrast, the process is totally inappropriate for producing parts which have a non-flat abrasion-proof surface, i.e. a surface which has grooves and ribs. Producing a ceramic or carbon mould is a much too complex and expensive operation and means that this process cannot be used industrially.